This invention relates to a method for obtaining nuclear magnetic resonance information data from a target, particularly a method for obtaining nuclear magnetic resonance information caused by chemical shift, which may be utilized for an apparatus for medical diagnosis.
A prior art apparatus for obtaining a space distribution of nuclear magnetic resonance (hereinafter referred to as NMR) spectrum data from a target and displaying the data as an image, has a construction shown in FIG. 1, for instance. Magnetic field generating means 11 generates a magnetic field Ho parallel to a Z-axis. Magnetic gradient field generating means 12 selectively generates magnetic gradient fields Gx, Gy and Gz parallel to the Z-axis and with the strength varying in the directions of X-, Y- and Z-axes, respectively. A target 13 is placed in a space, in which the magnetic field Ho and magnetic gradient fields Gx, Gy and Gz are selectively superimposed on one another. The axes, along which the strengths of the magnetic gradient fields Gx, Gy and Gz vary, need only cross one another and need not be perpendicular to one another.
A radio frequency (hereinafter referred to as RF) pulse current is caused through a coil 14 to apply an RF electromagnetic field to the target 13. An NMR signal which is generated from the target 13 at this time, is received by the coil 14. The received signal is fed to an RF signal transmitter/receiver 15 for amplification and detection. The detected signal is fed to an A/D converter 16 to be sampled for conversion into a digital signal which is in turn fed to a signal processor 17 consisting of an electronic computer. The signal processor 17 provides a processed NMR signal which is displayed as an image on a display 18.
Using the apparatus as described above, the NMR signal is collected by the so-called spin-warp method, for instance, in the following sequence.
To obtain the space distribution of NMR data of the target, a main magnetic field Ho which has a uniform magnetic intensity distribution in the Z-axis direction is applied to the target 13. At the same time, a Z-axis magnetic gradient field Gz with the strength thereof varying in the Z-axis direction (hereinafter referred to as Z-axis gradient field), as shown in FIG. 2, is applied. Further, a so-called 90.degree. RF pulse is applied in parallel to the Y-axis direction normal to the Z-axis. Thus, the nuclei of a particular type of atoms, e.g., hydrogen atoms, in a slice of the target 13 intersecting a certain point on the Z-axis resonate to the 90.degree. RF pulse under the given Z-axis main magnetic field Ho, and also their nuclear spins are tilted to lie in the X-Y plane to cause Lamor precession in the X-Y plane about the Z-axis, thus causing nuclear spin excitation in a selected slice 13a of the target 13. Subsequently, the Z-axis gradient field Gz is removed, and a Y-axis gradient field Gy is applied for a short period of time. As a result, the phase of the nuclear spins on lines intersecting the Y-axis at respective points in parallel with the X-axis are shifted from line to line according to the strengths of the Y-axis gradient field on the lines. Thereafter, an X-axis gradient field with the strength thereof varying in the X-axis direction is applied as a readout magnetic gradient field Gxs. A free induction signal which is obtained at this time is detected. The detected signal is then subjected to Fourier transformation with respect to time, whereby NRM data at each point on the X-axis is obtained as data at a different frequency. Usually, when the Y-axis gradient field Gy is provided for a short period of time an inverted Z-axis gradient field Gzr and an inverted X-axis gradient field Gxd are provided for rephasing and dephasing, respectively, in that period.
The above sequence of operations, which is labeled Sp in FIG. 3, is repeatedly performed for a necessary resolution by successively changing the gradient field strength of the Y-axis gradient field Gy. A series of the signals, which are obtained for the respective strengths of the Y-axis gradient field as a result of the Fourier transform noted above, are further subjected to Fourier transformation, thereby obtaining NMR data on Y-axis in the slice 13a. In other words, a two-dimensional Fourier transform is performed on a series of the signals which are obtained by repeatedly executing the sequence Sp shown in FIG. 3. In this way, the slice 13a is divided into, for instance, 256 by 256 pixels, and NMR data is available for each pixel. In this case, the sequence Sp noted above is performed 256 times. The repeated sequences Sp with various strengths of the Y-axis gradient field Gy are usually illustrated in a superimposed relation with one another on the same time axis as shown in FIG. 4.
It is well known that the frequency of free induction signal obtained from the same atom varies slightly depending on the state of chemical bonding thereof. This phenomenon is called chemical shift. For example, both water (H.sub.2 O) and lipid (either--CH.sub.2 --or--CH.sub.3 --) in the given slice contain hydrogen atoms, but the NMR frequency varies by 3.2 ppm between the hydrogen atom of water and that of lipid. Therefore, if the NMR frequencies of these two hydrogen atoms, one in water and the other in lipid, in a given slice are separately detected, it is possible to display as an image the distributions of water and lipid.
When obtaining NMR data from a living body, it is necessary to take the following into considerations.
(a) If the target is moved during the data collection, it deteriorates the obtained image quality. Therefore, the target should be held as stationary as possible. However, if the measurement is continued for a long time, it gives pain to the target, i.e., a patient for instance, thus frequently resulting in a movement of the target. For this reason, the measurement time is desirably as short as possible.
(b) To detect chemical shift information from the target, it is desirable that the existing NMR computer tomograph system (NMR-CT) can be utilized without alteration of its hardware structure. Further, it is desired that no excessive energy is projected onto the target.
United Kingdom Patent Application Publication GB No. 2,143,041 A discloses a method for obtaining NMR data including chemical shift data. In this method, as shown in FIG. 5, a slice of the target is excited by applying a gradient field Gz and a 90.degree. RF pulse, and then Y- and X-axis gradient fields Gy and Gx are applied for a short period of time. Then, after a period .tau. a so-called 180.degree. RF pulse is applied to the coil 14 shown in FIG. 1 to rotate the direction of the excited nuclear spins by 180 degrees. Subsequently, Z- and Y-axis gradient fields Gz and Gy are applied, and then X-axis gradient field Gx is applied as read-out field. The period T.sub.1 from the excitation of the slice till the second application of the gradient fields Gzr and Gy, that is, till the application of the readout field Gxs, is set to be constant. The purpose of variation in strength of Y-axis gradient field Gy after the 180.degree. RF pulse application is the same as that of the variation in strength of Y-axis gradient field Gy shown in FIG. 3. With respect to each strength of the Y-axis gradient field Gy the generation of the 180.degree. RF pulse is shifted by a period .DELTA..tau..
When the slice is excited by the 90.degree. RF pulse, the directions, i.e., the phases, of the excited nuclear spins coincide with one another, and the detected free induction signal has the highest level. However, with the lapse of time the phases of the nuclear spins disperse to reduce the free induction signal level. When the 180.degree. RF pulse is applied after a lapse of time .tau., the direction of each nuclear spin is inverted, that is, those spins with lagging phases relative to a reference are converted into the ones with leading phases, while those with leading phases are converted into the ones with lagging phases. If the dephasing gradient field Gxd is not applied, a gradually decaying free induction signal FI would be observed and, after application of the 180.degree.-pulse, a gradually increasing spin echo (or so-called Hahn echo) signal SE, which reaches a maximum after a lapse of time since the 180.degree.-pulse, between the application of the 90.degree. RF pulse and the 180.degree. RF pulse by .DELTA..tau. while holding constant readout period T.sub.1, the timing of reaching the maximum level of the spin echo signal SE would be caused to shift by 2.DELTA..tau. as indicated by the two-dotted chain line. However, the application of the dephasing gradient field Gxd causes rapid degeneration of the NMR signal and after a lapse of time 2.DELTA..tau., a gradient echo signal GE can be obtained. By varying the interval between the 90.degree. pulse and the 180.degree. pulse, the peak value of the GE signal will decrease in accordance with the level of the shifted spin echo signal SE'. Since the NMR frequency varies due to a chemical shift, the phase of Lamor precession of the nuclear spin varies to an extent corresponding to the frequency variation due to the chemical shift during the time 2.DELTA..tau.. That is, after lapse of the time 2.DELTA..tau., the phases of spins having larger chemical shifts are in greater advance. For example, under conditions of Gy=0 and .tau.=.tau..sub.1, before application of readout gradient field Gxs the frequencies, represented by .omega. and .DELTA..omega., and phases, represented by directions of arrows, of the nuclear spins of hydrogen atoms in CH.sub.2 and hydrogen atoms in OH at the respective positions along the X-axis are as shown in FIG. 6A. As shown, the phase and frequency .omega. of the spin are identical with all the hydrogen atoms in CH.sub.2 and also the phase and frequency .omega.+.DELTA..omega. of the spin are identical with all the hydrogen atoms in OH. When the readout gradient field Gxs is applied in this state, the nuclear spin frequencies of hydrogen atoms in CH.sub.2 are .omega., .omega.+.DELTA..omega., .omega.+2.DELTA..omega., . . . and the nuclear spin frequencies of hydrogen atoms in OH are .omega.+.DELTA..omega., .omega.+2 .DELTA..omega., .omega.+3.DELTA..omega., . . . at X-axis positions of increasing values as shown in FIG. 6B. In other words, components of identical frequency are obtained at each two adjacent positions on the X-axis, so that it is impossible to discriminate the hydrogen atom of CH.sub.2 and that of OH against one another.
Then, it is set such that .tau.=.tau..sub.2 and .tau..sub.2 &lt;.tau..sub.1, and 2(.tau..sub.1 -.tau..sub.2) causes the phase of the hydrogen atom nuclear spin in OH to lead the phase of the hydrogen atom nuclear spin in CH.sub.2 by .pi. as shown in FIG. 6C. When the readout gradient field Gxs is applied in this state, the frequencies of the hydrogen atom nuclear spins in CH.sub.2 are .omega., .omega.+.DELTA..omega., .omega.+2.DELTA..omega., . . . while the frequencies of the hydrogen atom nuclear spins in OH are .omega.+.DELTA..omega., .omega.+2.DELTA..omega., .omega.+3.DELTA..omega., . . . at successive X-axis positions as shown in FIG. 6D. Besides, the nuclear spins of the hydrogen atoms in CH.sub.2 are in phase while the nuclear spins of the hydrogen atoms in OH are 180.degree. out-of-phase with those in the case shown in FIG. 6B, respectively. Thus, by adding signals obtained in the states shown in FIGS. 6B and 6D, the nuclear spin data of hydrogen atoms in OH are cancelled each other, and only nuclear spin data of hydrogen atoms in CH.sub.2 can be obtained. Also, by subtracting the signals obtained in the states shown in FIGS. 6B and 6D from one another, the nuclear spin data of hydrogen atoms in CH.sub.2 can be cancelled, and only those of hydrogen atoms in OH can be obtained. This means that NMR data containing chemical shift data can be obtained in the sequence shown in FIG. 5. While two different kinds of chemical shift data are obtained in the above case, a large number of different kinds of chemical shift data can be obtained by increasing the number of times of causing the change in the interval .tau. between the 90.degree. RF pulse and the 180.degree. RF pulse.
Where 16 different kinds of chemical shift data are to be measured, the change in time .tau. by .DELTA..tau. is caused 16 times. In this case, for 256 different strengths of the Y-axis gradient field, the measurement is carried out 256 by 16 times, and it requires a comparatively short time.
With the existing NMR-CT there have been proposed various sequences for obtaining NMR data, e.g., SR (saturation-recovery) sequence, SE (spin echo) sequence, IR (inversion recovery) sequence and multi-slice sequence where desired ones of the above sequences are appropriately combined. In the SE sequence, the signal level is maximum when time Te called echo time is Te=2.tau.. Therefore, with NMR-CT using the SE sequence usually Te and .tau. are fixed so that the maximum signal level can be obtained at all time. The prior art method of obtaining chemical shift data shown in FIG. 5 utilizes the SE sequence. In this case, however, it is necessary to control the interval .tau. between 90.degree. RF pulse and the 180.degree. RF pulse. This means that the method shown in FIG. 5 can not be carried out by merely altering program with the existing NMR-CT which uses the SE sequence where .tau. is fixed. Rather, it is necessary to remodel hardware as well. In other words, the prior art method shown in FIG. 5 can not be readily applied to the existing NMR-CT.